1. Field of the Invention
The present invention relates to a drive mechanism interposed between two member guides for linear movement relative to each other to move the two members relative to each other, and a movable table unit provided with the drive mechanism.
2. Description of the Related Art
A movable table unit applied to a machine tool, such as a machining center, will be described with reference to FIGS. 20 and 21. FIG. 20 is a front elevation of a movable table unit 7 used in a machine tool, and FIG. 21 is a sectional view taken on line XXI-XXI in FIG. 20. The movable table unit 7 includes a table 3 that is for supporting a workpiece and that is mounted on a base 1 fixedly installed on the floor, and linear guide members 5 extended on the base 1 to guide the table 3 for linear movement on the base 1. Generally, a drive mechanism including a rack 101 and a pinion 103 or a drive mechanism using a ball screw is used for moving the table 3.
Referring to FIGS. 20 and 21, a drive mechanism includes a rack 101 fixed to the top surface of the base 1, and a pinion 103 rotatably supported on the table 3 and engaged with the rack 101. The pinion 103 is driven to rotate by a motor 105 and the like, and the teeth provided on the circumference of the pinion 103 are meshed with the linear teeth of the rack 101. According to this structure, the table 3 is made to move linearly. (Refer to, for example, “Kikai Kogaku Binran, New Edition”, The Japan Society of Mechanical Engineers, B1-108, May 15, 1988.) The drive mechanism transmits force through the engagement of the teeth of the rack and the pinion. Therefore, a stress path, i.e., a path of force transmission from the table 3 to the base 1, is short and, hence the rigidity of the drive mechanism is high; that is, the force transmission path does not extend over the entire length of the rack 1, and the drive mechanism has high-rigidity.
Another drive mechanism will be described with reference to FIGS. 22 and 23.
FIG. 22 is a side elevation of a drive mechanism including a ball screw, and FIG. 23 is a partly cutaway perspective view of the drive mechanism shown in FIG. 22.
The drive mechanism includes a threaded rod 111 having axially opposite end journals 111a and 111b supported by bearings on a base 1, and a threaded nut 113 fastened to a table 3 and engaged with the threaded rod 111. The threaded nut 113 moves together with the table 3 along the threaded rod 111 when the threaded rod 111 rotates. As shown in FIG. 23, the threaded rod 111 is linked to the nut 113 by a plurality of balls 115 constrained in the space formed by helical grooves formed in the peripheral surface of the threaded rod 111 and the inner peripheral surface of the threaded nut 113. When the threaded rod 111 is driven for rotation, the balls 115 roll while bearing axial load, and the nut 113 is moved axially to move the table 3 linearly relative to the base 1. (Refer to, for example, “Kikai Kogaku Binran, New Edition”, The Japan Society of Mechanical Engineers, B2-173, Sep. 30, 1991.) In this drive mechanism, backlash between the threaded rod 111 and the threaded nut 113 can be reduced by axially pressurizing the balls 115 in advance to prevent play between the threaded rod 111 and the threaded nut 113. Therefore, the drive mechanism is able to position the table 3 accurately.
Those known drive mechanisms, however, have the following common problems.
(1) The stroke of the drive mechanism for the table 3 cannot easily be changed. The stroke for the table 3 moved by the former drive mechanism is dependent on the length of the rack 101, and that of the stroke for the table 3 moved by the latter drive mechanism is dependent on the length of the threaded rod 111. Thus, the stroke of the table 3 cannot easily be changed after the installation of the drive mechanism.
(2) The rack 101 needs to be replaced with a new one when a part of the teeth of the rack 101 is broken, and the threaded shaft 111 needs to be replaced with a new one when part of the thread of the threaded rod 111 is broken. Therefore, these drive mechanisms are poor in maintainability.
Further, those known drive mechanisms have the following individual problems.
(1) Although the former drive mechanism is satisfactory in rigidity, there is a backlash between the mating teeth of the rack 101 and the pinion 103. Inevitably, the backlash causes play between the rack 101 and the pinion 103, and hence the former drive mechanism is inferior to the latter drive mechanism in accuracy of positioning the table 3. Since the teeth of the rack 101 and the pinion 103 are in sliding contact and not in rolling contact, the teeth are liable to be abraded. Therefore, the drive mechanism is unsatisfactory in durability, and is also unable to operate quietly at high operating speeds.
(2) Although the latter drive mechanism is superior to the former in positioning accuracy, the stress path from the table 3 to the base 1 changes according to the stroke of the table 3. As shown in FIG. 22, force is transmitted from the table 3 to the base 1 through the threaded nut 113, the threaded rod 111 in engagement with the threaded nut 113, and the end journals 111a and 111b. Thus, the length of the stress path is dependent on the length of the threaded rod 111. Therefore, if the drive mechanism has a long stroke, the stress path is long and the rigidity of the drive mechanism is very low.